Since there have been a few questions about the fuel indicating systems during this thread that may not have been answered, I thought an overall review might help.There are three fuel indicating systems on the Columbia aircraft:
  1. The fuel gauges
  2. The low-fuel annunciators
  3. The fuel totalizer on the MFD.

The Fuel Gauges


The fuel gauges have two fuel sensors per tank, one inboard and one outboard. These sensors are float type variable-resistor (potentiometer) sensors. They indicate the amount fuel with a changing voltage drop across the sensor. The two sensors are connected in series, which effectively adds the indications of the two sensors together.

The float type sensors have historical reliability problems that apply to all aircraft that use them. I've owned Cessnas, Beechcraft, and Cirrus aircraft, and they all have had a finicky fuel sensor or two. It was not uncommon to see in the older Columbia a flaky fuel sensor that took a little while to burn in.

However, in the pre-G1000 Columbia aircraft, the fuel gauges were analog fuel gauges--basically voltmeters that showed the voltage of the fuel senders as gallons of fuel. These gauges were less susceptible to noise on the line (minor voltage variations), and it would take a seriously flakey sensor to cause them to indicate problems.

The G1000 system has a analog-to-digital (A-to-D) converter which takes the fuel sensor voltage and converts it to a digital value. This A-to-D converter appears to be more susceptible to noise and passes that noise through to the indicating system. By increasing the base voltage through the sensors, Columbia will be increasing the signal-to-noise ratio (the ratio between the correct voltages and minor voltage variations). This sounds like a promising way to address the problem. This is also consistent with the fact that it is usually the inboard sensors that start showing problems--sensors that are in effect at lower total voltages.

The G1000 has a couple of warnings that are derived from the fuel level sensors. The first is that the fuel level indicator on the MFD will turn YELLOW when the fuel level drops below a certain amount. The second is that if the fuel levels indicate more than a 10 gallon imbalance, you will get a Message from the system.

Low Fuel Annunciators


The low-fuel annuciation system is separate from the fuel level indicating system. The low-fuel annuciator system has a separate switch down near the slosh box of each wing that will close when the fuel drops below approximately 8 gallons of usable fuel.

When this occurs, you will get a low-fuel annunciation on the PFD. This is separate and distinct from the fuel level indicators turning yellow.

The Fuel Totalizer


The fuel totalizer monitors the fuel flow input from the fuel transducer and keeps a running total of how much fuel is remaining. The accuracy of this depends on two things:
  1. How accurate was the initial setting
  2. How accurate is the fuel transducer

The only time you really know how much fuel is on board a stock aircraft is when you level the plane and fill the tanks. I have seen swings up +/- 10 gallons just by not having the plane level while fueling. Another typical problem is the time it takes for fuel to flow down to the lower portions of the tank. If you fill the tank and walk away, five minutes later it will be able to hold another 5 gallons or so. The recommended procedure is to fill one side, fill the other side, go back and top off the first side, then go back and top off the second side.

If you fill to the tabs, you should have at least 25 gallons per side. This is approximate. On some aircraft, I have seen it be more like 30, depending on manufacturing variance and how level the aircraft is.

I see a lot of pilots relying on the fuel gauges to initialize the fuel totalizer. The gauges are calibrated at two points: empty and full. Anything in between is an approximation. And if you are using the fuel totalizer to back up suspect fuel gauges, don't initialize it with suspect data.

If you *really* want to know how much fuel you have in each tank below full settings, then you need to calibrate where your tabs are and create your own calibrated stick. Empty the tank (run it dry), then add 4 gallons of unusable fuel. Now it is at zero usable fuel. Get a brand new, generic paint stirring stick from a hardware store. Add 5 gallons at a time to the tank, and mark the fuel levels on the stick. The tabs have a nice slot in them to allow a paint stick to go all the way down. As you add fuel to the tank, note the level of the tabs for your aircraft.

Empirical evidence suggests the accuracy of the fuel flow transducer is pretty good, but the specifications are such that it is acceptable for that transducer to be accurate +/- more than 1 gph. If your tranducer is off, the fuel totalizer will accumulate more and more error as time goes on. If you are in the habit of simply incrementing the fuel on board by the amount of fuel you purchased, you will not have reset that accumulated error. Periodically you need to set your fuel on board to an accurate value measured through some other means (full tank, calibrated tabs, or calibrated stick).

I hope this helps.

Peter King, MCFI
Bend, OR